California Encephalitis has been found to be an endemic disease in Ohio since its discovery in Ohio in 1964, with a range of 16 to 36 serologically confirmed causes occurring annually. Ohio, therefore, furnishes an excellent opportunity to study the epidemiology of this disease with particular emphasis on delineating these factors present which cause this disease to occur endemically with respect to vectors, reservoir hosts, and other ecological factors involved in the epidemiology. The significance of sero-positive conversions in humans will be determined. The nature, duration and significance of sequelae to California Encephalitis will be further defined. The incidence of unreported and/or subclinical infections will be studied in the general population, in both endemic and control areas, and within normal and mentally retarded populations. The infection ratios of various species of arthropods will be ascertained. Transmission studies to discover the most important vectors will be accomplished. The ability of various suspect reservoir hosts to develop viremia and the significance of this in transmission will be examined. The significance of sero-positive conversion in naturally occurring wild and domestic animal populations will be investigated. A search will be made to find naturally occurring sentinels. A means of recognition of endemic foci will be investigated with an aim of developing adequate control in localized areas.